The electron microscope has an extremely high resolution compared with optical microscopes, and is widely used in order to observe a detailed structure of an object to be observed clearly. The electron microscope acquires an image of an object sample by irradiating an electron beam onto the object sample and detecting particles (electrons of the same kind or of a different kind of the irradiated electrons, or electric magnetic wave, or photons) emitted from the object sample or penetrating the object sample with a detector. Observation objects of the electron microscope include materials, semiconductors, food, biotechnology, medical fields, etc.
In inspections and analyses using the electron microscope, there is a demand of wishing to check a whole image of a structure and its distribution in a wide range and there is a need of wishing to perform observation in a captured image whose visual field is wide with a large number of pixels (hereinafter referred to as a wide field image). In addition, also in the wide field image, a high-image-quality image, i.e., an image of a high image quality, with a high S/N ratio, and of a small drift, is required.
As common imaging methods of the electron microscope, there are a high-speed (frame integration) scanning mode whereby an image is acquired by integrating two or more pieces of image data of the same visual field acquired by scanning at high speed and a low-speed scanning whereby one piece of image data of an excellent S/N ratio is acquired by scanning at a low speed. Although in the case of the high-speed scanning, there is little influence of the drift of a stage or a beam, in the case of the low-speed scanning, distortion of an image occurs because the imaging position of the structure in the image shifts during the scanning. Meanwhile, although a high-speed scanning mode has small distortion, since dislocation between frames cannot be eliminated, if the images are integrated as they are, there will be a problem that a combined image will be blurred. For this reason, conventionally, an imaging method of re-imaging a region that is wished to be observed in detail at a high magnification after acquiring the whole image at a low magnification is taken; it poses problems that the imaging requires much labor and time.
As means coping with these problems, there is proposed a more advanced imaging method whereby the wide field image of a high image quality is imaged. As the advanced imaging method, there is an imaging method that uses both the high-speed scanning mode and the low-speed scanning mode. For example, Patent Literature 1 describes an imaging method whereby a reference image without image distortion is acquired at a high-speed scanning mode and a captured image is acquired at a low-speed scanning mode for the same region as the reference image at the same magnification, and the captured image is corrected on the basis of the reference image. Patent Literature 2 describes an imaging method whereby a captured image of a low magnification is acquired at a high-speed scanning mode, a captured image of a high magnification is acquired at a low-speed scanning mode, and a reduced image of a region-of-interest image is combined to the captured image.